It is well known that various heterogeneous catalysts may be used to hydrogenate unsaturated compounds which comprise one or more unsaturated bonds. Most commercial heterogeneous catalysts, however, are poorly characterized materials which have a distribution of active sites which exhibit variability in their nature, activity, and selectivity.
Despite these drawbacks, there are often significant process advantages accrued when a heterogeneous catalyst is employed. Homogeneous catalysts, which offer uniform composition and reactivity, and often fewer by-products, can present difficulties in separating product from the catalyst (an especially important point if the catalyst is based on a precious metal) and may have increased energy requirements. The advantages of both modes of reaction would be offered if one would attach a homogeneous catalyst to an insoluble backbone, a process termed heterogenization.
Heterogenization of a homogeneously active catalyst is not a new idea; quite a bit of work has already been carried out on this concept, but the previous supports have most often consisted of chemically-modified inorganic and organic polymers. These materials often suffer from low chemical and thermal stability, (due to the reactivity of the support), have a variety of active sites (like a conventional heterogeneous catalyst) due to the geometric irregularity of the polymer, and often exhibit leaching of the catalyst into the product stream, affording a separation problem similar to that of homogeneous catalysis. The layered zirconium phosphonates and analogous compounds (especially the layered species) have been found to be suitable support materials for compositing with active hydrogenation catalysts selected from the group consisting of the metals of Group VIII of the Periodic Table of the Elements and salts and other compounds thereof. Although the Group VIII metals are preferred for hydrogenation activity, it is appreciated that the interaction between the support and metal as dealt with here would be applicable to hydrogenation active metals in other groups. The layered structure of such zirconium phosphonates and analogous compounds provide uniform catalytic sites, and may be chemically modified to minimize leaching. More demanding conditions could be applied without degradation of the support. In addition, the two-dimensional geometry of the interlayer space may result in an enhanced selectivity towards entering reactants or product formation.